Field of the Disclosure
The present disclosure relates generally to wireless communication systems and, more particularly, to coordinated multipoint communication in wireless communication systems.
Description of the Related Art
User equipment can improve downlink throughput by canceling or suppressing interference caused by downlink signals transmitted to other user equipment by neighboring cells. The user equipment can demodulate the downlink signals based on reference signals included in the downlink signals and then remove the demodulated signals from the signals received by the user equipment. Examples of reference signals include cell-specific reference signals (CRS) that are derived from a physical cell identifier of the transmitting cell, channel state information reference signals (CSI-RS) that are configured by the transmitting cell, and demodulation reference signals (DMRS) that are derived from a cell identifier such as the physical cell identifier. The reference signals for neighboring cells can be transmitted to user equipment by the user equipment's serving cell. For example, each cell may receive information identifying the configuration of reference signals for a set of neighboring cells over an interface such as a backhaul interface and may transmit information identifying the reference signals to user equipment when the user equipment connects to the serving cell, e.g., by handing off to the serving cell. The interfering cell reference signals that are signaled to the user equipment by its serving cell are a subset of the parameters used in network-assisted interference cancellation and suppression (NAICS).
Wireless communication systems may implement coordinated multipoint (CoMP) communication that allows user equipment to receive signals from its serving cell and one or more other cells using a virtual cell identifier instead of the physical cell identifier that identifies its serving cell. User equipment can be configured with one or more virtual cell identifiers and different cells can use the virtual cell identifiers to transmit to the user equipment. For example, a virtual cell identifier can be assigned to a first cell and a second cell and the virtual cell identifier can be provided to user equipment, which may be attached or connected to the first cell. Either the first cell or the second cell may then transmit signals to the user equipment using the virtual cell identifier. In some cases, both the first cell and the second cell may concurrently transmit signals to the user equipment using the virtual cell identifier. The transmitting cell or cells may change from subframe to subframe. Transmission by the first or second cells is transparent to the user equipment because the user equipment does not need to know the source of the signal. Reference signals such as the DMRS may be derived from the virtual cell identifier and user equipment may use the DMRS to demodulate the signals transmitted to the user equipment by the first or second cells using the virtual cell identifier. However, other (non-virtual) reference signals such as the CRS and CSI-RS are not derived from the virtual cell identifier and may therefore be different for the first cell and the second cell.
User equipment can identify the non-virtual reference signals (such as the CRS and CSI-RS) using a mapping between the virtual cell identifier and the non-virtual reference signals. For example, a quasi-co-location indicator can be used to inform user equipment that different cells are co-located. The user equipment can use the non-virtual reference signals from one of the co-located cells in combination with virtual reference signals (such as the DMRS) to demodulate downlink signals transmitted by other co-located cells. However, in CoMP communication, not all of the cells associated with a virtual cell identifier are necessarily co-located and consequently the virtual cell identifier does not uniquely indicate the non-virtual reference signals that should be used by user equipment to demodulate downlink signals from cells associated with the virtual cell identifier. The mapping of the virtual cell identifier to the non-virtual reference signals may therefore be implemented as a static mapping between the non-virtual reference signals and a dynamic mapping between the virtual cell identifier and one of the non-virtual reference signals. For example, the static mapping may indicate different combinations of a CRS and a CSI-RS for the cells associated with the virtual cell identifier and the dynamic mapping may indicate which combination is associated with the virtual cell identifier during the current subframe. The static mapping may be provided to the user equipment at connection, e.g., using radio resource control (RRC) signaling of information transmitted over a backhaul interface, and the dynamic mapping may be provided to the user equipment in the downlink control information (DCI) transmitted in the subframe that includes the non-virtual reference signals. Thus, the dynamic mapping may change from subframe to subframe.
The virtual cell identifier can be transmitted over a backhaul interface to neighboring cells so that user equipment connected to the neighboring cells can identify the virtual reference signals for interference suppression or cancellation. However, transmitting the dynamic mapping over the backhaul interface is impractical because the delays in the backhaul interface are typically long relative to the duration of a subframe. Thus, user equipment may be limited to demodulating interfering signals using virtual reference signals such as the DMRS during CoMP communications. Although this may be sufficient for interference suppression or cancellation for relatively large transmissions that include the DMRS in three or more physical resource blocks (PRBs) of the subframe, it is not sufficient for smaller transmissions of one or two PRBs in the subframe. One proposal is to remove the need for dynamic mapping by requiring that a virtual cell identifier be associated with a single set of non-virtual reference signals that can be specified by the static mapping. However, this limits the flexibility of CoMP communication provided by the virtual cell identifiers.